You ask in your initial post how to get good, useful information from a reader... After your, "I really tore into them. Mainly because they did have good ideas but they were executed very sloppily. One girl wrote a deeply personal autobiographical piece almost entirely in the passive voice, where she made a life changing decision with the powerful, moving line:"The decision had been made."" quote...
With an attitude like that and especially with writers who have exposed their inner thoughts to strangers, even if badly expressed, shows a huge lack of empathy on your part. Now, if you have your own cut-throat variety type group--have at it. These poor folks were probably just taking an English credit. Lighten up, my friend. Different groups have different goals and different levels of how they crit each other.
I think you need to pull back a bit. I might even apologize... When a writer is using her personal experiences, she is exploring her inner soul. Not all writers write for others. And writers have various levels of proficiency. In a new group or in a forced class crit group, your goal is to share what is right first. Then pick one or two things that, in your opinion, might improve the work and something that their skill level can accomplish with work. Writing is a journey that never ends. Your own work will grow and develop over your lifetime. What you can't see now, you might with time.
If you enjoy tearing apart someone's work in the manner that you describe and in the setting that you describe? I find that rather sadistic my friend. :-) I'm pretty sure that you didn't really express yourself in that manner, right?
My favorite quote, "Could have laughed it off and done less damage." Not a bad way to approach a critique.